Examples of a radiation analyzing apparatus capable of discriminating radiation energy include an energy dispersive spectroscopy (hereinafter, referred to as EDS) and a wavelength dispersive spectroscopy (hereinafter, referred to as WDS).
The EDS is an X-ray detector of a type that converts the energy of X-rays taken into a detector into an electrical signal in the detector and calculates the energy by the magnitude of the electrical signal. In addition, the WDS is an X-ray detector of a type that monochromatizes (energy discrimination) X-rays using a spectroscope and detects the monochromatized X-rays using a proportional counter tube or the like.
As the EDS, semiconductor detectors such as a silicon lithium (SiLi) type detector, a silicon-drift type detector, and a germanium detector are known. For example, a silicon lithium type or silicon-drift type detector is often used in an element analyzing apparatus of an electron microscope, and can detect energy in a wide range from approximately 0.2 keV to 20 keV. However, since silicon is used for the detector, the properties thereof, in principle, depend on a band gap (approximately 1.1 eV) of silicon, and thus it is difficult to improve an energy resolution to equal to or greater than approximately 130 eV, and the energy resolution becomes lower by 10 times or more than that of the WDS.
In this manner, the wording “energy resolution which is one of indexes indicating the performance of an X-ray detector is, for example, 130 eV” means that energy can be detected by uncertainty of approximately 130 eV when the X-ray detector is irradiated with X-rays. Therefore, as the uncertainty becomes lower, the energy resolution becomes higher. That is, when characteristic X-rays constituted by two adjacent spectrums are detected, uncertainty becomes lower as an energy resolution becomes higher. When a difference in energy between two adjacent peaks is approximately 20 eV, it is possible to, in principle, separate the two peaks from each other by an energy resolution of approximately 20 eV to 30 eV.
In recent years, superconductive X-ray detectors, which are energy dispersive type detectors, having the same energy resolution as that of a WDS have attracted attention. Among these superconductive X-ray detectors, a detector including a superconductive transition edge sensor (hereinafter, referred to as a TES) is a high-sensitivity calorimeter using a sharp resistance change (for example, a resistance change is 0.1Ω when a temperature change is several mK) when a metal thin film transitions from a superconductive state to a normal conductive state. Incidentally, the TES is also referred to as a micro calorie meter.
The TES analyzes a sample by detecting a temperature change occurring within the TES when fluorescent X-rays or characteristic X-rays generated from the sample by radiation irradiation with primary X-rays, primary electron beams, or the like are incident thereon. The TES has an energy resolution higher than those of other detectors, and can obtain an energy resolution of, for example, equal to or less than 10 eV in characteristic X-rays of 5.9 keV.
When a TES is installed in a scanning electron microscope or a transmission electron microscope, characteristic X-rays generated from a sample irradiated with an electron beam are acquired by the TES, and thus it is possible to easily separate peaks of energy spectrums of characteristic X-rays (for example, Si-Kα, W-Mα, or W-Mβ) which are not separable by a semiconductor type X-ray detector.
Incidentally, in an X-ray analyzing apparatus adopting the superconductive X-ray detector, a superconducting quantum interference device (hereinafter, referred to as a SQUID) amplifier is used to read out an extremely small current change in the TES. In order to realize a high energy resolution of the TES, it is important to keep a current flowing to the SQUID amplifier constant. This is because a change in a current flowing to the SQUID amplifier has to be reduced in order to obtain a high energy resolution, as described later.
As an apparatus for keeping a current flowing to the SQUID amplifier, that is, a base line current flowing to the TES constant, there is known, for example, an X-ray analyzing apparatus that corrects, when the base line current flowing to the TES deviates from a fixed value and fluctuates, a current flowing to the TES or a wave height value based on the current in accordance with the fluctuation width thereof (see JP 2009-271016 A).
In addition, there is known a radiation analyzing apparatus that corrects a wave height value of a signal pulse of a TES on the basis of correlation between an output of a heater embedded into a pedestal having the TES installed thereon and a base line current flowing to the TES (see JP 2014-38074 A). The radiation analyzing apparatus acquires in advance characteristics of the correlation between the output of the heater and the sensitivity of the TES, and corrects the wave height value of the signal pulse of the TES using the sensitivity of the TES corresponding to the output of the heater when a signal pulse of the TES is acquired during the actual measurement.
However, in the above-mentioned X-ray analyzing apparatus and radiation analyzing apparatus, in a case where errors of detection values increase in each of the base line current and the output of the heater immediately before the actual measurement, or the like, the using of the detection values results in a concern that it is not possible to appropriately correct a signal pulse.